“Although children who sucked their thumb or bit their nails were slightly less likely to have a reaction to the skin tests, when the habits were looked at individually only thumb sucking was linked to a skin test reaction at 13 – and neither habit individually for skin tests at 32.”

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The study, published in the medical journal Pediatrics, looked at data collected from 1,037 children born in Dunedin, New Zealand, in 1972 to 1973.

Parents were asked about their child’s thumb sucking and nail biting habits when they were five, seven, nine and 11 years of age.

They were asked if the statements “frequently sucks their finger/thumb” or “frequently bites their nails” applied to their child “not at all”, “somewhat” or “certainly”.

Sensitivity to allergies was tested by skin prick tests for various substances including dust mites, grass, animal fur, and wool.

However, the NHS stated: “This study does not provide good evidence that thumb sucking or nail biting have any effect on a child’s likelihood of developing allergies.

“There were also no clear links for any specific allergic reaction – and no links at all with reported asthma or hay fever.

“So this doesn’t give a clear answer of whether these habits are linked with allergy risk or not.”

They noted further limitations, including the “subjective nature” of the parental reports and the fact the study looked at children born in a single city more than 40 years ago.

“Thumb sucking or nail biting are common childhood habits,” the NHS concluded.

“Most children grow out of them and they are only usually considered a problem requiring treatment if they persist once a child has started school.”

Guidance on Break Thumb Sucking, a website created by an orthodontist dedicated to helping parents put an end to their child’s habit, states parents should understand the triggers for a child sucking their thumb and use a progress chart.

“Attempt to tackle the root causes,” the website states. “Common reasons for sucking thumbs include anxiety, boredom, tiredness and self-soothing.

“A progress chart is a great incentive for your child and makes them an active participant in the project.

“Reward them for the times they remember to stop sucking their thumb as it will increase their willingness to break the habit.”